Critical dimension analysis with simultaneous multiple angle of incidence measurements

ABSTRACT

A method and apparatus are disclosed for evaluating relatively small periodic structures formed on semiconductor samples. In this approach, a light source generates a probe beam which is directed to the sample. In one preferred embodiment, an incoherent light source is used. A lens is used to focus the probe beam on the sample in a manner so that rays within the probe beam create a spread of angles of incidence. The size of the probe beam spot on the sample is larger than the spacing between the features of the periodic structure so some of the light is scattered from the structure. A detector is provided for monitoring the reflected and scattered light. The detector includes multiple detector elements arranged so that multiple output signals are generated simultaneously and correspond to multiple angles of incidence. The output signals are supplied to a processor which analyzes the signals according to a scattering model which permits evaluation of the geometry of the periodic structure. In one embodiment, the sample is scanned with respect to the probe beam and output signals are generated as a function of position of the probe beam spot.

PRIORITY

This application of U.S. application Ser. No. 10/150,032, filed May 17,2002, now U.S. Pat. No. 6,654,131, which is in turn a continuation ofU.S. application Ser. No. 09/818,703, filed Mar. 27, 2001, now U.S. Pat.No. 6,429,943, which claimed priority to provisional application Ser.No. 60/192,899, filed Mar. 29, 2000.

TECHNICAL FIELD

The subject invention relates to optical metrology equipment formeasuring critical dimensions and feature profiles of periodicstructures on semiconductor wafers. The invention is implemented usingdata obtained from simultaneous multiple angle of incidence measurementsas an input to analytical software designed to evaluate surface featuresvia a specular scatterometry approach.

BACKGROUND OF THE INVENTION

There is considerable interest in the semiconductor industry inevaluating small features of periodic structures on the surface of asample. In current high density semiconductor chips, line widths orfeature sizes are as small as 0.1 microns. These feature sizes are toosmall to be measured directly with conventional optical approaches. Thisis so because the line widths are smaller than the probe beam spot sizewhich can be achieved with most focusing systems.

This problem is illustrated in FIG. 1 which shows a wafer 10 havingformed thereon a number of conductive lines 12. A probe beam 14 is shownfocused by a lens 20 onto the sample at a spot 16. The reflected beam ismeasured by a photodetector 18. As can be seen, spot 16 overlapsmultiple lines 12 and therefore cannot be used to measure distancesbetween lines or the thickness of the lines themselves.

To overcome this problem, sophisticated software programs have beendeveloped which analyze the reflected probe beam in terms of ascattering model. More specifically, it is understood that criticaldimensions or feature profiles on the surface of the wafer will causesome level of scattering of the reflected probe beam light. If thisscattering pattern is analyzed, information about the criticaldimensions can be derived. This approach has been called specularscatterometry. The algorithms use various forms of modeling approachesincluding treating the lines as an optical grating. These algorithmsattempt to determine the geometry of the periodic structure.

FIG. 2 schematically illustrates the geometry of one type of periodicstructure 24. This periodic structure can be analyzed in terms of thewidth W between the features and the depth D of the grooves. Inaddition, the shape or profile P of the side walls of the features canalso be analyzed by the current algorithms operating on the analyticaldata.

To date, these analytical programs have been used with data taken fromconventional spectroscopic reflectometry or spectroscopic ellipsometrydevices. In addition, some efforts have been made to extend thisapproach to analyzing data from simultaneous multiple angle of incidencesystems. In these systems, the spot size is relatively small, but stilllarger than the individual features of the periodic structure.Paradoxically, where the features are only slightly smaller than thespot size, analysis through scatterometry is difficult since not enoughof the repeating structure is covered by the spot. Accordingly, it wouldbe desirable to modify the system so a sufficient number of individualfeatures are measured so a good statistically based, scatterometryanalysis can be performed.

SUMMARY OF THE INVENTION

The assignee of the subject invention has previously developedsimultaneous multiple angle of incidence measurement tools which havebeen used to derive characteristics of thin films on semiconductorwafers. It is believed that data from the same type of tools can be usedwith an appropriate scattering model analysis to determine criticaldimensions and feature profiles on semiconductors.

Detailed descriptions of assignee's simultaneous multiple angle ofincidence devices can be found in the following U.S. Pat. Nos.4,999,014; 5,042,951; 5,181,080; 5,412,473 and 5,596,411, allincorporated herein by reference. The assignee manufactures a commercialdevice, the Opti-Probe which takes advantage of some of thesesimultaneous, multiple angle of incidence systems. A summary of all ofthe metrology devices found in the Opti-Probe can be found in PCTapplication WO/9902970, published Jan. 21, 1999.

One of these simultaneous multiple angle of incidence tools is marketedby the assignee under the name beam profile reflectometer (BPR). In thistool, a probe beam is focused with a strong lens so that the rays withinthe probe beam strike the sample at multiple angles of incidence. Thereflected beam is directed to an array photodetector. The intensity ofthe reflected beam as a function of radial position within the beam ismeasured and includes not only the specularly reflected light but alsothe light that has been scattered into that detection angle from all ofthe incident angles as well. Thus, the radial positions of the rays inthe beam illuminating the detector correspond to different angles ofincidence on the sample plus the integrated scattering from all of theangles of incidence contained in the incident beam. In this manner,simultaneous multiple angle of incidence reflectometry can be performed.

Another tool used by the assignee is known as beam profile ellipsometry.In one embodiment as shown and described in U.S. Pat. No. 5,042,951, thearrangement is similar to that described for BPR except that additionalpolarizers and/or analyzers are provided. In this arrangement, thechange in polarization state of the various rays within the probe beamare monitored as a function of angle of incidence.

It is believed that the data generated by either of these tools could beused to appropriately model and analyze critical dimensions and featureprofiles on semiconductors.

The lens used to create the probe beam spot from a laser source in theabove two simultaneous multiple angle of incidence systems is typicallylarger than the distance between adjacent features of the periodicstructure of interest. However, in order to provide statisticallysignificant information, it is desirable that information be collectedfrom at least twenty or more of the repeating features. One method ofachieving this goal is to increase the spot size of the probe beam. Suchan approach is described in U.S. Pat. No. 5,889,593 incorporated byreference. In this patent, a proposal is made to include an opticalimaging array for breaking up the coherent light bundles to create alarger spot.

It is believed the latter approach is not desirable because of theadditional complexity it introduces into the measurement. Ideally, whenattempting to analyze a periodic structure (e.g., a periodic criticaldimension array) it is desirable to have no additional periodicities inthe measurement system between the source and detector. Multipleperiodic signals are more difficult to analyze and are often plaguedwith added uncertainty and ambiguity with respect to extractingparameters associated with any of the constituent components.

In accordance with the subject invention, the requirement for increasingthe area over which measurements are taken is achieved in two differentways. In the first approach, the probe beam spot is scanned over thewafer until a sufficient amount of data are taken. Once the data aretaken, a spatial averaging algorithm is utilized. Spatial averaging isdiscussed in U.S. patent application Ser. No. 09/658,812, filed Sep. 11,2000 and incorporated herein by reference.

In another approach, the probe beam is generated by an incoherent orwhite light source. When incoherent light is focused by a lens, the spotsize will be significantly larger than with a laser. No separate imagingarray needs to be included to break up the coherence of the light as inthe prior art. In such a system, a monochrometer could be locatedbetween the light source and the detector to permit measurement of anarrow band of wavelengths. The wavelength selected can be matched tothe type of sample being inspected in order to obtain the moststatistically relevant data. In addition, it would also be possible toscan the monochrometer in order to capture data at multiple wavelengths.It would also be possible to measure multiple wavelengths simultaneouslyas described in U.S. Pat. No. 5,412,473.

Alternatively or in addition, the measurement data which can be obtainedfrom two or more metrology devices of the type described in the aboveidentified PCT application, could be used to advance this analysis. Asmore of these metrology devices are added, the ability to unambiguouslydistinguish features increases. Thus, it is within the scope of thesubject invention to utilize either or both of a simultaneous multipleangle of incidence spectrometer or ellipsometer along with one or moreof spectroscopic reflectometry, spectroscopic ellipsometry or absoluteellipsometry tools with the latter two being deployed in a manner thatmaximizes the information content of the measurement. For example, witha rotating compensator spectroscopic ellipsometer one measures both thesign and magnitude of the ellipsometric phase while in more standardconfigurations, e.g., a rotating polarizer/rotating analyzer, only themagnitude or phase can be measured.

An example of an analytical approach for evaluating critical dimensionsusing data from a broadband reflectometer is described in “In-situMetrology for Deep Ultraviolet Lithography Process Control,” Jakatdaret. al. SPIE Vol. 3332, pp. 262-270 1998. An example of using aspectroscopic ellipsometer equipment for CD metrology is described in,“Specular Spectroscopic Scatterometry in DUV Lithography, SPIE Vol.3677, pp 159-168, from the SPIE Conference on Metrology, Inspection andProcess Control for Microlithography XIII, Santa Clara, Calif., March1999.

Further and related information measuring critical dimensions can befound in U.S. Pat. Nos. 5,830,611 and 5,867,276, incorporated herein byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic diagram illustrating the optical measurement ofperiodic structure on a sample.

FIG. 2 is a cross-sectional illustration of the type of periodicstructure which can be measured in accordance with the subjectinvention.

FIG. 3 is a schematic diagram of an apparatus for performing the methodof the subject invention.

FIG. 4 is a schematic diagram illustrating an alternate embodiment ofthe subject apparatus for performing spectroscopic measurements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning to FIG. 3, a basic schematic of simultaneous multiple angle ofincidence apparatus 30 is illustrated. Further details about such adevice are described in U.S. Pat. Nos. 4,999,014; 5,042,951; 5,159,412and 5,412,473 all incorporated herein by reference. As noted above, theassignee's Opti-Probe device incorporates portions of this technologyand markets the measurement subsystem under the trademark Beam ProfileReflectometry or BPR. In the past, the BPR technology was utilizedprimarily to analyze the characteristics of thin films formed onsemiconductors. This disclosure is directed to using the measurementswhich can be obtained from this type of system to evaluate the geometryof periodic structures formed on semiconductors.

The basic measurement system includes a light source 32 for generating aprobe beam 34. The light source can be a laser for generating a coherentbeam of radiation. Laser diodes are suitable laser sources for thisapplication. If the output of the laser is not itself polarized, aseparate linear polarizer can be provided. As discussed below, lightsource 32 can also be a polychromatic or white light source forgenerating a probe beam with a plurality of wavelengths.

The probe beam 34 is focused onto the sample 10 using a lens 40 in amanner so that the rays within the probe beam create a spread of anglesof incidence. In the preferred embodiment, the beam is directed normalto the surface but can be arranged off-axis as illustrated in U.S. Pat.No. 5,166,752, incorporated by reference. Lens 40 is preferably a highnumerical aperture lens (on the order of 0.90) to create angles ofincidence from zero to about 70 degrees. The lens creates rays havingpredominantly S-polarized light along one axis and predominantlyP-polarized light along an orthogonal axis. At intermediate angles, thepolarization is mixed.

Lens 40 is positioned to create a probe beam spot 42 on the sample onthe order of about 1 micron in diameter where the light source iscoherent (i.e. a laser source). This spot is typically somewhat largerthan the spacing (width W) between the recurring features of theperiodic structure. For this reason, a certain portion of the light fromthe probe beam will be diffracted or scattered from the periodicstructure. As discussed below, this light can be analyzed with ascattering model in a manner similar to prior art probe beam detectionscatterometry systems. The advantage of the subject approach is that thedata can be simultaneously obtained from a plurality of angles ofincidence.

In order to obtain data sufficient to perform an accurate evaluation, itis preferable that the probe beam collect information from at least 20repeating features in the pattern. If the probe beam spot 42 is notsufficiently large, than it would be desirable to scan the probe beamover the surface of the sample in the region of the periodic structure.This can be accomplished by moving an X-Y stage 44 upon which the samplerests. It would also be possible to provide scanning capability to theprobe beam itself. Scanning would preferably be in a directionperpendicular to the parallel features of the periodic structure. Datais generated as a function of the position of the probe beam spot withrespect to the features of the periodic structure. Where the sample isscanned, the data is analyzed as discussed above and further clarifiedusing a spatial averaging algorithm. In this spatial averaging approach,the data from points in the scan are filtered by a repeated sequence ofaveraging and outlier exclusions where the outliers are defined by theirdifferences with respect to signal levels and symmetry properties. Theresult of this process leads to data that are equivalent to those takenwith an incoherent source illuminating an area the same as that scannedin the spatial averaging approach.

The reflected/scattered beam passes back up through the lens 40 whichcollimates the beam. The reflected beam is redirected by a splitter toan imaging lens 48. Lens 48 magnifies and relays an image of the sampleat the focal plane of the lens. A spatial filter 50 having an apertureis placed in the focal plane of the lens 48 for controlling size of thearea of the sample which is measured.

The probe beam is than passed through a 50—50 splitter and directed totwo photodetectors 54 and 56 having a linear array of detector elements.The photodetectors are arranged orthogonal to each other to measure boththe S and P polarization components. As described in detail in the abovecited patents, each of the detecting elements in the array measuredifferent angles of incidence. The radial position within the reflectedprobe beam is mapped to the angle of incidence, with the rays closer tothe center of the beam having the smallest angles of incidence and therays in the radially outer portion of the beam corresponding to thegreatest angles of incidence. Thus, each detector element simultaneouslygenerates an independent signals that correspond to a different angle ofincidence.

The output signals from the detector arrays are supplied to theprocessor 60. Processor will analyze the signals based on algorithmwhich considers the reflected and scattered light such as a rigorouscoupled wave analysis. The selected algorithm will correlate thevariation in reflectivity as a function of angle of incidence with thegeometry of the periodic structure. Such scattered light theoreticalmodels are well known in the literature. In addition to the articlescited above, further examples can be found in the following articleswhich are cited by way of example. Those skilled in the art of analyzingsignals diffracted from periodic structures will understand that thereare many other approaches which can be utilized. It should be noted thatsince this approach obtains measurements at multiple angles ofincidence, higher order diffraction effects may be collected andconsidered.

Prior Articles:

1. “Optical Etch-Rate Monitoring: Computer Simulation of Reflectance,”Heimann and Schultz, J. Electrochem. Soc: Solid State Science andTechnology, April 1984, Vol. 131, No. 4, page 881

2. “Optical Etch-Rate Monitoring Using Active Device Areas: LateralInterference Effects”, Heimann, J. Electrochem. Soc: Solid State Scienceand Technology, August 1985, Vol. 132, No. 8, page 2003.

3. “Scatterometry for 0.24 micron-0.70 micron developed photoresistmetrology,” Murnane et. al. SPIE, Vol. 2439, page 427 (1995).

4. “Multi-Parameter Process metrology using scatterometry,” Raymond et.al. SPIE Vol. 2638, page 84 (1995).

5. “Specular Spectral Profilometry on Metal Layers,” Bao et. al, SPIEVol 3998 (2000), page 882.

The type of analysis will depend on the application. For example, whenused for process control, either in situ or near real time, theprocessor can compare the detected signals to an expected set of signalscorresponding to the desired geometry of the periodic structure. If thedetected signals do not match the expected signals, it is an indicationthat the process is not falling within the specified tolerances andshould be terminated and investigated. In this approach, nosophisticated real time analysis of the signals is necessary.

As is known in the art, the reflected output signals at multiple anglesof incidence can be more rigorously analyzed to determine the specificgeometry of the periodic structure. While there are a number ofdifferent approaches, most have certain traits in common. Morespecifically, the analytical approach will typically start with atheoretical “best guess” of the geometry of the measured structure.Using Fresnel equations covering both the reflection and scattering oflight, calculations are made to determine what the expected measuredoutput signals would be at different angles of incidence for thetheoretical geometry. These theoretical output signals are compared tothe actual measured output signals and the differences noted. Based onthe differences, the processor will generate a new set of theoreticaloutput signals corresponding to a different theoretical periodicstructure. Another comparison is made to determine if the theoreticalsignals are closer to the actual measured signals. These generation andcomparison steps are repeated until the differences between thetheoretically generated data and the actually measured data aresubstantially minimized. Once the differences have been minimized, thetheoretical periodic structure corresponding to the best fit theoreticaldata is assumed to represent the actual periodic structure.

This minimization procedure can be carried out with a conventional leastsquares fitting routine such as a Levenberg-Marquardt algorithm. Itwould also be possible to use a genetic algorithm. (See, U.S. Pat. No.5,953,446.)

In the past, this type of rigorous analysis was limited to the researchenvironment, since the calculations necessary to determine the periodicstructure was extremely complex and time consuming. However, with adventof faster and parallel processing technologies, it is believed that suchan analytical approach could be used in a real time analysis.

One method for reducing the computer processing time during measurementactivities is to create a library of possible solutions in advance. (Seethe Jakatdar articles, cited above). In this approach, a range ofpossible periodic structures and their associated theoretical outputsignals are generated in advance using the Fresnel equations asdiscussed above. The results are stored as a library in a processormemory. During the measurement activities, the actual measured signalsare compared with sets of theoretically generated output signals storedin the library. The periodic structure associated with the set oftheoretical signals which most closely matches the actual measured datais assumed to most closely represent the geometry of the measuredperiodic structure.

The simultaneous multiple angle approach is not limited toreflectometry. As noted in U.S. Pat. Nos. 5,042,951 and 5,166,752(incorporated herein by reference), it is also possible to obtainellipsometric measurements corresponding to ψ and Δ simultaneously atmultiple angles of incidence. To obtain such measurements, someadditional optical elements should be added to the device of FIG. 3. Forexample, a polarizer 66 (shown in phantom) is desirable to accuratelypredetermine the polarization state of the probe beam. On the detectionside, an analyzer 68 (also shown in phantom) is provided to aid inanalyzing the change in polarization state of the probe beam due tointeraction with the sample. The optical components of the analyzer canbe of any type typically used in an ellipsometer such as a polarizer ora retarder. The ellipsometric output signals are analyzed in a fashionsimilar to the prior art approaches for using ellipsometric data toevaluate the geometry of periodic structures.

Another approach to increasing the size of the probe beam spot is to usean incoherent source for the probe beam. Such an incoherent source caninclude a variety of well known spectral line or broad band sources. Ifa spectral line light source is used, some modest level of narrow passfiltering may be desirable. Such a filter could be located either beforethe sample or before the detector as indicated in phantom lines 69 a and69 b. The wavelength which is used is selected in order to maximize thesensitivity in the reflection response to the type of changes ofinterest.

It would also be possible to use a broadband or white light sourcegenerating a polychromatic beam. In this situation, the wavelengthselective filter could be in the form of a conventional monochrometer. Amonochrometer, which typically includes a dispersive element and a slit,functions to transmit a narrow band of wavelengths. The system could bearranged to take measurements at only one wavelength or in a series ofsequential wavelengths as the monochrometer is tuned. The use of anincoherent light source would fill the field of view on the sample(typically 100 microns or more for a 0.9 NA microscope objective). Theactual measurement spot size is controlled by an aperture that can bevaried in size as needed for the particular measurement in question.Such variable spatial filtering is described in U.S. Pat. No. 5,412,473.

It would also be possible to set up a system where both the multipleangle and multiple wavelength information is obtained simultaneously.Such a detection scheme is also described in detail in U.S. Pat. No.5,412,473, incorporated by reference. This detection scheme is brieflydescribed herein with reference to FIG. 4.

In this embodiment, the probe beam 34 a is a broadband polychromaticbeam generated by a white light source 32 a generating an incoherentprobe beam. There are a number of white light sources available such astungsten or deuterium bulbs. The probe beam 34 a is focused on thesample with lens 40. Upon reflection, the probe beam is passed throughrelay lens 48 and spatial filter 50 in the manner described above. Inaddition, the beam is passed through a filter 70 having an slit 72located in the relay image plane of the exit pupil of lens 40. Lens 48also serves to relay this image. The slit is dimensioned so that imagetransmitted to the detector 74 will be on the order of the dimensions ofa row of detector elements 76.

After the beam passes through the slit, it is dispersed as a function ofwavelength by element 80. Any conventional wavelength dispersing elementcan be used, such as a grating, prism or holographic plate.

The dispersed beam is directed to the detector which is a twodimensional array of photodiodes. A CCD element could also be used. Theslit 72 is oriented perpendicular to the axis of the dispersion of thelight. In this matter, each horizontal row of elements on the array 74will measure a narrow wavelength band of light. Each of the elements ineach row correspond to different angles of incidence. Thus, the outputof the detector 74 will simultaneously produce data for multiplewavelengths and multiple angles of incidence. As noted in U.S. Pat. No.5,412,473, this type of detection system can be used for eitherreflectometry or ellipsometry measurements.

It is also within the scope of the subject invention to combine thesemeasurements with other measurements that might be available from acomposite tool. As noted above, the assignee's Opti-Probe device (asdescribed in WO 99/02970) has multiple measurement technologies inaddition to the Beam Profile Reflectometry system. These othertechnologies include broadband reflectometry and broadband ellipsometry.The output from these additional modules can be used in combination withthe BPR signals to more accurately evaluate the geometry of the periodicstructures.

In summary, there has been described a method and apparatus forevaluating relatively small periodic structures formed on semiconductorsamples. In this approach, a light source generates a probe beam whichis directed to the sample. In one embodiment, the light source generatesincoherent light. A lens is used to focus the probe beam on the samplein a manner so that rays within the probe beam create a spread of anglesof incidence. The size of the probe beam spot on the sample is largerthan the spacing between the features of the periodic structure so someof the light is scattered from the structure. A detector is provided formonitoring the reflected and scattered light. The detector includesmultiple detector elements arranged so that multiple output signals aregenerated simultaneously and correspond to multiple angles of incidence.The output signals are supplied to a processor which analyzes thesignals according to a scattering model which permits evaluation of thegeometry of the periodic structure. Both single and multiple wavelengthembodiments are disclosed.

While the subject invention has been described with reference to apreferred embodiment, various changes and modifications could be madetherein, by one skilled in the art, without varying from the scope andspirit of the subject invention as defined by the appended claims.

We claim:
 1. A method for monitoring the processing of semiconductorwafers, wherein the processing creates one or more geometrical featureson the surface of the wafer having at least one dimension significantlyless than a micron, said method comprising the steps of: focusing acoherent probe beam of radiation to a spot overlapping the feature onthe sample surface in a manner so that the rays within the probe beamcreate a spread of angles of incidence and wherein the spot size on thesample is on the order of one micron in diameter so that the probe beamis diffracted upon reflection; monitoring the diffracted probe beamlight and simultaneously generating a plurality of independent outputsignals corresponding to a plurality of different angles of incidence;and comparing the output signals to an expected set of signals todetermine if the process is within a specified tolerance.
 2. A method asrecited in claim 1, wherein said process is terminated if the outputsignals are outside said specified tolerance.
 3. A method as recited inclaim 1, wherein said probe beam is focused onto the sample while thesample is in situ.
 4. A method as recited in claim 1, wherein the probebeam is generated by a laser.
 5. A method as recited in claim 1, whereinthe probe beam is passed through an analyzer and wherein the change inpolarization state of the rays within the probe beam are monitored.
 6. Amethod as recited in claim 1, wherein the expected set of signals aregenerated based on a theoretical profile of the feature and whereinafter the comparison step, another set of expected signals are generatedbased on the results of the comparison using a different theoreticalprofile of the feature and wherein the comparison and generation stepsare repeated until the differences between the expected set of signalsand the output signals are minimized.
 7. A method as recited in claim 1,wherein the during the comparison step, the output signals are comparedto a set of previously generated theoretical expected signals to findthe closest match and wherein each one of the set of previouslygenerated expected signals corresponds to a different possible geometryof the feature.